**Core Concept**
DMPA, or depot medroxyprogesterone acetate, is a long-acting reversible contraceptive injection that primarily works by inhibiting ovulation through the action of medroxyprogesterone acetate on the hypothalamic-pituitary axis. It also has effects on the endometrium and cervical mucus.

**Why the Correct Answer is Right**
Medroxyprogesterone acetate in DMPA causes a decrease in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secretion, which ultimately leads to inhibition of ovulation. The endometrium becomes atrophic due to the lack of estrogenic and progestogenic stimulation, making it unfavorable for implantation of a fertilized ovum. Additionally, DMPA causes thickening of cervical mucus, which prevents sperm penetration.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because DMPA does cause a decrease in menstrual bleeding, but this is not an exception to its effects. Instead, it is a well-documented side effect due to the progestogenic effect on the endometrium.
**Option B:** Incorrect because DMPA can cause weight gain in some users, although the exact mechanism is not fully understood. It may be related to changes in appetite or metabolism.
**Option C:** Incorrect because DMPA can cause a decrease in bone density, particularly in adolescents and young adults. This is thought to be due to the suppressive effect of DMPA on estrogen levels, which are essential for maintaining bone health.

**Clinical Pearl / High-Yield Fact**
It is essential to inform users of DMPA that it may take several months for the full contraceptive effect to become apparent, and that users should use an additional form of contraception during this time.

**Correct Answer: A.**